1. Field of the Invention
The invention relates to air drying and separation operations. More particularly, it relates to the production of both dry air and nitrogen products.
2. Description of the Prior Art
Permeable membrane processes and systems have been proposed and used increasingly for the production of nitrogen by air separation. In such operations, feed air is brought into contact with the surface of the membrane, and oxygen, as the more readily permeable component of air, passes through the membrane while nitrogen, the less readily permeable component of air, is withdrawn from the membrane system as a non-permeate product stream. Conventional membrane systems typically produce nitrogen having Purity levels of up to about 99.5%. Higher nitrogen purities can be achieved if a post-purification feature is employed. Such processing may also include the use of a countercurrent membrane dryer with permeate gas from an air separation membrane being employed as purge gas in the dryer membrane. In some industrial operations, a source of clean, dry compressed air is also desired to prevent corrosion and condensation in instrumentation, piping, pneumatic tools, ventilators and other plant operations. For such latter purposes, separate adsorptive or refrigerative dryers have typically been utilized. Such units, however, tend to have significant product losses, due to purge and blowdown steps, or generally require a significant expenditure of added energy, e.g. for regeneration or refrigeration.
The ability of various membrane materials to permeate moisture is well known in the art. Thus, a membrane system utilizing such a material would be used to replace the function of the adsorptive (or regenerative) dryer for producing clean, dry air for such variety of purposes. However, many practical commercial asymmetric and composite-type membranes are characterized by the crossflow permeation model performance, wherein the composition of the bulk gas on the permeate side of the membrane has little effect on the permeation rates achieved. Accordingly, the removal of moisture from compressed air streams by means of such membranes requires the co-permeation of significant amounts of valuable product gas as well. Operation at stage cuts, i.e., the permeate to feed flow ratio, on the order of 10% to 30% might be required to achieve the dew points typically Provided by adsorptive or refrigerative dryers. As a result, the substitution of crossflow-type membrane dryers to produce clean, dry compressed air is not economically attractive.
Advances in membrane product development and theory have resulted in the recognition in the art that some composite and asymmetric membranes are characterized by a significant degree of countercurrency in operation, and thus follow, at least in part, the countercurrent permeation mathematical model of the art rather than the crossflow model. Membranes designed so that permeation therethrough to a significant extent can be described by the countercurrent model can benefit from the use of purge gas on the low pressure permeate side of the membrane. It is possible, in such circumstances, to achieve lower stage cuts to achieve the same moisture removal level as in crossflow permeation designs, provided that dry purge gas is available at the job site.
While the membrane drying art has thus progressed and membrane air separation systems have been employed in commercial operations, it is desirable to achieve further advances in the art, Particularly in light of the ever increasing performance requirements applicable to membrane systems in satisfying the needs of industrial applications. In particular, it is desirable in the art that membrane systems be provided for the enhanced integration of membrane air separation and air drying applications.
It is an object of the invention to provide improved air treatment membrane systems and processes.
It is another object of the invention to provide membrane systems and processes suitable to provide both air separation and air drying operations.
It is a further object of the invention to provide improved membrane systems and processes for the production of both nitrogen product and dry air.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.